1. Field of the Invention
The present invention relates generally to technologies for indexing a risk of severe stress symptoms triggered by flashing, colorful lights displayed on video display terminal (VDT) such as a television, a computer terminal, etc.
2. Description of Related Art
With the recent developments of computer graphics techniques including virtual reality, a variety of images are displayed on a video screen of a television, computer terminal, etc., and the bright flashing lights of a a TV cartoon (for example) became a serious matter of triggering seizures or seizure-like symptoms. It is reported that flickers or flashes at a rate ranging from 2 to 55 Hertz is liable to induce seizures, various convulsions, vomiting, irritated eyes and other symptoms in certain photosensitive people. It is further reported that there is some evidence that flickering outside the 2–55 Hertz range can also cause seizures.
One approach to reducing or preventing such stress symptoms triggered by violent flickering or flashing is disclosed in the Laid-opened Japanese Patent Application No. 8-286653 (first prior art). According to this prior art, the video image signals whose brightness changes at about 10 Hertz are adaptively reduced using a temporal low-pass filterer and displaying the temporal frequency-treated images on the screen. However, this prior art suffers from the difficulties that the images displayed after being temporally filtered are deteriorated due to the reduction of frequency components of the moving pictures.
In order to overcome the above-mentioned problem, it has been proposed to generate a risk index signal in connection with the video signals which might cause the sever stress symptoms such as seizures. This proposal is disclosed in Japanese Patent Application P2001-154648A (second prior art) filed by the same entity as the assignee of the present application, in accordance with which upon detecting the video images which are likely to induce the sever stresses, an alarm or a warning signal is issued to alert viewers, Accordingly, it is possible to avoid the undesirable deterioration of the reproduced images.
Before turning to the present invention, it is deemed preferable to describe the second prior art with reference to FIG. 1.
FIG. 1 is a diagram schematically showing functional blocks for generating a VDT seizure risk index and relevant part of television receiver set.
In the following, the severe stress symptoms may be referred to as VDT seizures or simply seizures for the convenience of descriptions.
A primary-color signal generator 10 is supplied with a luminance signal Y and three color-difference signals R-Y, G-Y, and B-Y. The generator 10 produces three primary-color signals R, G, and B which are applied to a display unit (not shown). In addition, the primary-color signals R, G, and B are respectively applied to signal processing sections 12, 14, and 16. The outputs of the sections 12, 14, and 16 are fed to a VDT seizure risk index generator 18.
The signal processing sections 12, 14, and 16 are identical with one another in terms of the circuit configurations and the functions thereof, and as such, only the section 12 is illustrated in detail in FIG. 1. The section 12 generally comprises an analog-to-digital (A/D) converter 20, a field memory 22, and a temporal low-pass filter 24 which in turn comprises three field memories 26, 28, and 30, and an arithmetic circuit 32.
The field signals (analog) of primary-color R are applied to the analog-to-digital (A/D) converter 20 and converted to the corresponding digital field signals thereat without distinction between adjacent fields. The A/D converter 20 applies the digital field signals to the field memories 22 and 26 one by one.
In the interlaced scanning, one frame consists of two fields (odd and even fields), and the frame rate is about 30 per second and the field rate (or flash rate) is about 60 per second in the NTSC (the National Television System Committee) color transmission system. The arithmetic circuit 32 retrieves a field signal from the field memory 26 and also retrieves the previous field signal from the field memory 28, and calculates new pixel values based on the corresponding pixel values of the two-field signals obtained form the field memories 26 and 28 using the following equation (1)Ii′(t)=(1−δ)×Ii(t)+δ×Ii(t−Δt)  (1)where i denotes a two-dimensional coordinate used to represent the position of each pixel in the field, Ii(t) denotes a pixel value in the coordinate i at a time t, Δt denotes a time interval between two fields (Δt= 1/60 seconds in the NTSC color transmission system), and δ is a coefficient (0<δ≦1) which determines the characteristics of the temporal low-pass filter 24 and is typically set to 0.7 by way of example.
As shown in the above, the arithmetic circuit 32 operates such as to weight Ii(t) and Ii(t−Δt) by the coefficient (1−δ) and δ respectively and then adds the multiplication results. The new pixel values thus calculated over one whole field are successively cumulated in the field memory 30. It is understood that the low temporal frequency components over the successive fields can be determined from the cumulated pixel values.
The VDT seizure risk index generator 18 is supplied with the pixel data from the field memories 22 and 30. In addition, the generator 18 receives the pixel data similar to those stored in the field memories 22 and 30 from the signal processing units 14 and 16, and calculates a,seizure risk index e(t) using the following equation (2)
                              e          ⁡                      (            t            )                          =                                            ∑              c                        ⁢                                          ∑                i                            ⁢                                                w                  c                                ⁢                                                                                                                                                  I                          i                                                ⁡                                                  (                          t                          )                                                                    -                                                                        I                          i                          ′                                                ⁡                                                  (                          t                          )                                                                                                                          m                                                                          N            ×                                          (                                  I                  ⁢                                                                          ⁢                  max                                )                            m                        ×                                          ∑                c                            ⁢                              w                c                                                                        (        2        )            where
(1) wO represents weighting coefficients wR, wG, and wB (0<wR, wG, wB≦1) of respective primary-color signals R, G, and B;
(2) Ii(t) denotes a pixel value at a time t in the coordinate i with respect to the primary color signals R, G and B;
(3) Ii′(t) is determined using equation (1);
(4) Imax denotes the maximal value of each pixel, and in case each pixel is represented by 8 bits, Imax becomes 255;
(5) N denotes a total number of pixels in one field, and if one frame consists of 640×480, then N=15360(=640×480/2); and
(6) m is an index representing human's non-linear sensitivity as to the VDT seizure risk, and typically set to 1, 2, or 3.
If the weighting coefficients wR, wG, and wB are made different, it is possible to change the seizure risk index e(t) depending on the image colors. It is admitted that the red color flash is liable to induce the VDT stress symptoms, and as such, if the weighting coefficient WR is made large relative to wG, and wB, the red color flash can be reflected on the seizure risk index e(t). Further, the index e(t) is normalized in equation (2) by the total number of pixels in one field, and thus it is possible to reduce undesirable influence due to the differences of display screen sizes.
As mentioned above, the seizure risk index e(t) is determined based on the differences between the cumulated pixel values (stored in the field memory 30) and the pixel values of one field (stored in the filed memory 22), and accordingly, the second prior art suffers from the difficulty that the risk index e(t) erroneously indicates a high value due to white noise and/or contours of picture images irrespective of fact that the possibility of inducing the stress symptoms is extremely low. In other words, the second prior art is unable to correctly evaluate the VDT seizure risk based on the human's visual sensation.
More specifically, for example, in case white noise is superimposed on the original primary-color signals, the differences between the corresponding pixel data stored in the field memories 22 and 30 become large, resulting in erroneously indication of the seizure risk possibility.
By way of example, in the case where half of the pixels in the frames flashes, if the flashing pixels are randomly scattered over the frame, the seizure risk index e(t) indicates a high level irrespective of the fact that the VDT seizure risk is very low. On the contrary, in the same case, if the flashing pixels are concentrated on half of each of the frames, the index e(t) becomes high because the seizure risk is high. However, the second prior art is encountered the problem that the above-mentioned two cases are unable to be distinguished.
Further, it is often the case that the corresponding pixel values of the adjacent fields change abruptly at the contour of the picture images. In such an instance, the differences between the two field signals stored in the memories 22 and 30 become large, leading to an undesirable result of erroneously increasing the VDT seizure risk. The aforesaid prior art also suffers from the difficulty of being unable to deal with such a case.